For instance, a coal-fired power plant generates electric power by burning coal particles pulverized by a coal pulverizing apparatus with a furnace to produce a combustion gas, generating steam through heat exchange with the combustion gas, and driving a turbine by the steam.
The load of the coal-fired power plant is not always constant. The coal-fired power plant can be operated with the change in load. For instance, in a case where the coal-fired power plant is connected to a utility grid, it is desired to rapidly change the load of the coal-fired power plant in response to a demand of the utility grid, for stabilizing the utility grid frequency or other purposes.
In the coal-fired power plant, however, even if the amount of coal (raw material coal) supplied to the coal pulverizing apparatus is changed, there is a time lag (coal output delay) until the coal output, i.e., the amount of coal discharged from the coal pulverizing apparatus is changed. It is therefore difficult to rapidly change the load of the coal-fired power plant.
In this regard, Patent Document 1 discloses that the rotational speed of a table is determined based on a coal supply amount command value and a parameter related to the change in the load of a generator in order to overcome the coal output delay.
Patent Document 2 discloses a method for controlling a vertical mill, including changing the coal supply amount in accordance with an increase/decrease in load of the vertical mill and changing the rotational speed of a table to cover the lack or excess of the coal discharge amount due to the time delay between coal supply and coal discharge.
Patent Document 3 discloses that the coal supply amount and the rotational speed of a classifier are controlled in response to a load correction signal obtained based on dynamic characteristic of the coal discharge amount when the output command is changed followed by the change of parameters such as the moisture or the hardness of coal, a primary air flow rate, and the rotational speed of the classifier.
Patent Document 4 discloses a method for controlling a coal pulverizing apparatus, including subtracting an output demand signal from a signal obtained by inputting the output demand signal into a first-order lag operator to generate a correction signal, processing the correction signal by a limiter and an integrator, and adding a signal generated from a constant generator to generate a rotational speed command for a rotary separator (rotary classifier) in accordance with a load state. The constant generator is configured to set the rotational speed of the rotary separator (rotary classifier) to a fixed value.
Patent Document 5 discloses a method for controlling a coal pulverizing apparatus including a main operation circuit which calculates a command signal associated with the coal supply amount on the basis of detection data from a boiler or a generator, an additional control unit which calculates the deviation between a standard coal output pattern preset in the coal pulverizing apparatus and a current coal output pattern, in which a calculation result by the additional control unit is added to the main operation circuit as a correction signal.
Patent Document 6 discloses a pulverized coal supply system in which at least one manipulated value of a mill, a primary air conveying part, or a coal supply part is determined based on a coal output (coal discharge amount) determined based on driving conditions of the mill and output power necessary for a furnace.
Patent Document 7 discloses that even when the temperature of an outlet of a coal pulverizer is changed due to a control of the opening degree of a conveying-air-flow-rate adjustment damper at the change in load, a coal output temperature correction signal is determined based on the deviation between a detected temperature and a set temperature of the outlet of the coal pulverizer, and the coal output temperature correction signal is used for controlling the opening degree of the conveying-air-flow-rate adjustment damper to ensure the coal discharge amount in response to a coal discharge amount command signal.